Leg extensions they place more stress on your knees than squats
The pec deck It over stretch the front of the shoulder and cause the muscle at the back of the should to stiffen, this equal future shoulder problems
The smith machine Puts you in an unnatural position that puts a lot of stress on the lower back and knees,
And most importantly with machines you dont work your stabilizer muscles while the muscle gets stronger this can lead to injuries in the future, Free weights are the way to go, I know most of you are afraid because its a harder workout and you prefer the easy way of putting your lazy ass in a machine and pump out a few reps but you are fucking up your body and you dont even know it.